1. Field of the Invention:
The present invention relates to the field of power supplies and, more particularly, to step-down DC-DC converters or step-down switching regulators wherein controlling the primary and secondary output voltages is desired.
2. Description of the Related Art:
FIG. 1A shows a prior art step-down switching regulator 5 with a primary winding 6. Step-down switching regulator 5 includes primary winding 6, a capacitor 9 and two input switches S.sub.A 3 and S.sub.B 4 controlled by input signals D 7 and D 19, respectively. It should be noted that the signal D is a logical inversion of the signal D, according to the conventional signal naming provision. A step-down switching regulator such as the one shown in FIG. 1A is commonly used to convert a DC unregulated input voltage, V.sub.IN 2, to a regulated DC output voltage, V.sub.OUT 8. In this configuration, V.sub.OUT 8 is less than V.sub.IN 2, and hence, the name step-down is used. Switches S.sub.A 3 and S.sub.B 4 of step-down switching regulator 5 are used to alternatively connect the first side (23) of primary winding 6 to either V.sub.IN 2 or ground. The second side (25) of primary winding 6 is connected to V.sub.OUT 8. The regulation of V.sub.OUT 8 is achieved by controlling the duty cycle of the input switches--S.sub.A 3 and S.sub.B 4.
In most applications, S.sub.B 4 has been a rectifier rather than a switch. However, recent demand for higher conversion efficiencies and lower output voltages has forced the use of switches, also called as synchronous rectifiers, for S.sub.B 4.
Now referring to FIG. 1B, because many applications require more than one output voltage, a step-down switching regulator 25 is provided with a secondary winding 16 magnetically coupled to a primary winding 15. It should be noted that more windings can be added to generate more output voltages. A secondary output voltage, V.sub.SEC 20, developed across secondary winding 16 can be regulated because the voltage, V.sub.PRI 15, across primary winding 15 is known when S.sub.B 11 is on. As shown in FIG. 1C, V.sub.SEC 20 is equal to -W*V.sub.OUT when S.sub.A 10 is off and S.sub.B 11 is on, where W is the ratio determined by dividing the number of turns in secondary winding 16 by the number of turns in primary winding 15. The coupling direction is chosen such that V.sub.SEC 20 is the turns ratio W times V.sub.PRI.
Now referring to FIGS. 1B and 1C, Since (i) an input signal D 13 and an input signal D 14 are complementary, (ii) D controls S.sub.A 10, and (iii) D 14 controls S.sub.B 11, when S.sub.A 10 is on, S.sub.B 11 is off. V.sub.LX 21 is equal to V.sub.IN when S.sub.A 10 is on, and V.sub.LX 21 becomes zero when S.sub.B 11 is on. V.sub.PRI 22 stays at the voltage level of V.sub.IN -V.sub.OUT when S.sub.A 10 is on, and V.sub.PRI 22 becomes -V.sub.OUT when primary winding 15 is connected to ground through S.sub.B 11. Because of the coupling direction, V.sub.SEC 20 is W*V.sub.PRI. Thus, when V.sub.PRI 22 is V.sub.IN -V.sub.OUT, V.sub.SEC 20 becomes W*(V.sub.IN -V.sub.OUT), and when V.sub.PRI 22 is -V.sub.OUT, V.sub.SEC 20 is -W*V.sub.OUT.
A DC output voltage can be obtained by rectifying and filtering V.sub.SEC 20. FIGS. 2A-2D show various ways of connecting the output(s) of the secondary winding(s) to generate output voltages that are: (i) positive, (ii) positive above V.sub.OUT, (iii) negative and (iv) negative and positive.
FIG. 2A shows a prior art step-down switching regulator for generating a positive secondary output voltage. The positive side of a secondary winding 31 is coupled to a rectifier 32 and to an output voltage V.sub.A 30 to produce a positive output voltage at V.sub.A 30. The negative side of secondary winding 31 is connected to ground. The output voltage V.sub.A 30 is equal to the voltage V.sub.S across secondary winding 31 subtracted by V.sub.D across rectifier 32, or V.sub.A =V.sub.S -V.sub.D. When S.sub.B 35 is on, the voltage V.sub.P across a primary winding 33 is -V.sub.OUT. Since V.sub.S is M/N*(-V.sub.P), where M is the number of turns in secondary winding 31, and N is the number of turns in primary winding 33, V.sub.S is M/N*V.sub.OUT. Thus, V.sub.A 30 becomes M/N*V.sub.OUT -V.sub.D. Accordingly, the step-down switching regulator in FIG. 2A produces a positive output voltage.
Now referring to FIG. 2B, a prior art step-down switching regulator for generating a positive secondary output voltage above the output voltage of a primary winding is shown. The negative side of a secondary winding 41 is connected to V.sub.OUT 44, while the positive side of secondary winding 41 is coupled to a rectifier 42 and to an output voltage V.sub.A 40 to generate a positive voltage above V.sub.OUT at V.sub.A 40. In FIG. 2B, the output voltage V.sub.A 40 is equal to the sum of the voltage V.sub.S across secondary winding 41 and V.sub.OUT 44 subtracted by V.sub.D across rectifier 42, or V.sub.A =V.sub.S +V.sub.OUT -V.sub.D. Since V.sub.S is M/N*V.sub.OUT, V.sub.A 40 becomes M/N*V.sub.OUT +V.sub.OUT -V.sub.D, or V.sub.A =(M/N+1)*V.sub.OUT -V.sub.D. V.sub.A 40 is equivalent to the sum of V.sub.A 30 and V.sub.OUT 34. Accordingly, the step-down switching regulator in FIG. 2B produces a positive output voltage above V.sub.OUT 44.
FIG. 2C presents a prior art step-down switching regulator for generating a negative secondary output voltage. While the elements are connected in a similar manner as the ones in FIG. 2B, the coupling direction of a secondary winding 51 is reversed, and the direction of a rectifier 52 is reversed. In this example, a secondary output voltage V.sub.A 50 becomes the sum of the voltage V.sub.S across secondary winding 51 and the voltage V.sub.D across rectifier 52, or V.sub.A =V.sub.S +V.sub.D. Since V.sub.S is -M/N*V.sub.OUT, V.sub.A 40 becomes -(M/N*V.sub.OUT -V.sub.D). Thus, the step-down switching regulator in FIG. 2C produces a negative output voltage.
FIG. 2D is a prior art step-down switching regulator for generating a positive and negative secondary output voltages. The step-down switching regulator in FIG. 2D incorporates two secondary windings 61 and 71, each having M turns and L turns, respectively. Secondary winding 61 is connected in the same way as secondary winding 51 is FIG. 2C. However, while secondary winding 51 is coupled to V.sub.A 50 in FIG. 2C, secondary winding 61 is coupled to an output voltage V.sub.A 60 in FIG. 2D. Secondary winding 71 is connected in the same way as secondary winding 31 in FIG. 2A. Secondary winding 71 is coupled to V.sub.B 70 as secondary winding 31 is coupled to V.sub.A 30 in FIG. 2A. Therefore, V.sub.A 60, like V.sub.A 50, becomes -(M/N*V.sub.OUT -V.sub.D). Likewise, V.sub.B 70, like V.sub.A 30, becomes M/N*V.sub.OUT -V.sub.D. Accordingly, the step-down switching regulator in FIG. 2D produces both a negative output voltage and a positive output voltage.
A step-down converter is the most efficient converter topology. However, the efficiency factor can be degraded by various loss mechanisms listed below from the highest to the lowest loss:
1. Switch and inductor resistive losses; PA1 2. Transformer core losses; PA1 3. Switching losses; PA1 4. Controller element quiescent power.
Under medium to heavy load currents, loss mechanisms 1 and 2 dominate. However, at light load currents, loss mechanisms 1 and 2 are relatively low compared to loss mechanisms 3 and 4. Modern converters, therefore, reduce the switching frequency at light loads to minimize losses due to loss mechanism 3, and thereby improve light load efficiency.
Now referring again to FIG. 1B, the efficiency improvement at light load currents is typically achieved by skipping switching cycles of S.sub.A 10 and S.sub.B 11 entirely and delivering a predetermined amount of energy to a load during a cycle. Switch S.sub.A 10 is turned on for a predetermined amount of time, or until a packet of energy is delivered to the load. A control circuit (not shown) that controls S.sub.A 10 and S.sub.B 11 then goes into a low current mode, disables switching of S.sub.A 10 and S.sub.B 11 until V.sub.OUT 17 drops to a level that causes this process to be repeated. The process starts again by turning on S.sub.A 10.
Continuing to refer to FIG. 1B, this scheme has a drawback when multiple winding topology is incorporated. The control circuit can only regulate one of the outputs-the primary output coupled to primary winding 15 or the secondary output coupled to secondary winding 16--usually chosen to be the primary output. When the regulated primary output is lightly loaded, or not loaded at all, a feedback loop that couples the primary output to the control circuit turns on the high side switch such as S.sub.A 10 in FIG. 1B infrequently to preserve efficiency. This results in no power being delivered to the secondary output. If the secondary output is connected to a heavy load, the secondary output voltage collapses and cannot be regulated.
Another simple way to control a secondary output voltage in a step-down switching regulator may be alternatively pulsing switches such as S.sub.A 10 and S.sub.B 11 in FIG. 1B constantly so that energy is always delivered to the secondary output, rather than providing a feedback loop between the secondary output and a control circuitry. This scheme may seem effective, but has very low efficiency at light primary loads as indicated above.